This invention relates to a concentrated solution of p-phenylenediamine derivatives, e.g. of N-(2-methylsulphonylaminoethyl)-N-ethyl-3-methyl-p-phenylenediamine (CD-3) and of N-(2-hydroxyethyl)-N-ethyl-3-methyl-p-phenylenediamine (CD-4).
p-phenylenediamine derivatives, particularly the aforementioned compounds CD-3 and CD-4, are known developer substances for colour photographic silver halide materials. They are normally used as a concentrated solution in sulphuric acid. This acidic solution is very stable due to a small addition of sulphite. The free bases of p-phenylenediamine derivatives are very susceptible to oxidation, however, both in solution and in solid form.
If a sulphuric acid concentrate of p-phenylenediamine derivatives is neutralised with alkali hydroxides, precipitates are formed in the concentrate.
For use in one-part colour developer concentrates, however, neutralisation is absolutely necessary, since colour development only occurs under alkaline conditions. Therefore, the colour developer concentrate already has to be alkaline. In order to produce different colour developer formulations, there is therefore a need for a stable, alkaline p-phenylenediamine derivative which can be used universally.
The present invention thus relates to a concentrated, aqueous solution of a p-phenylenediamine derivative, characterised in that it
a) has a pH higher than 12.5,
b) contains 0.4 to 1.1 mol p-phenylenediamine derivative/litre,
c) contains 0.05 to 2 mol of an antioxidant/litre,
d) contains at most 35 % by weight of organic solvents with respect to the total solution,
e) contains at most 50 mmol sulphate ions/litre and
f) is single-phase.
The pH is preferably higher than 13.
This concentrated solution can be produced from the free base or from salts of the respective p-phenylenediamine derivative.
Examples of salts of the p-phenylenediamine derivative which can be used include phosphates, chlorides and sulphates. When sulphates are used, the sulphate is separated off, e.g. as an alkali sulphate (as described in EP 0 980 024, paragraph 58).
EP 0 980 024 describes a concentrated alkaline CD-3 solution which contains an antioxidant and which is low in sulphate. This concentrated solution consists of two phases. Phase separation is only suppressed if large amounts of ethylene glycol are added. A two-phase concentrate is unsuitable for the produktion of a colour developer concentrate.
Suitable water-soluble organic solvents include those from the series comprising glycols, polyglycols, alkanolamines, aliphatic and heterocyclic carbonamides, and aliphatic and cyclic monoalcohols.
Examples of suitable water-soluble solvents include derivatives of carboxylic acid amides and derivatives of urea such as dimethylformamide, methylacetamide, dimethylacetamide, N,Nxe2x80x2-dimethylurea, tetramethylurea, methanesulphonic acid amide, dimethylethylene-urea, N-acetylglycine, N-valeramide, isovaleramide, N-butyramide, N,N-dimethylbutyramide, N-(2-hydroxyphenyl)-acetamide, N-(2-meth-oxyphenyl)-acetamide, 2-pyrrolidinone, xcex5-caprolactam, acetanilide, benzamide toluenesulphonic acid amide, phthalimide;
aliphatic and cyclic alcohols e.g. isopropanol, tert.-butyl alcohol, cyclohexanol, cyclohexane-methanol, 1,4-cyclohexanedimethanol;
aliphatic and cyclic polyalcohols, e.g. glycols, polyglycols, polymer waxes, tri-methyl-1,6-hexanediol, glycerol, 1,1,1-trimethylolpropane, pentaerythritol, sorbitol;
aliphatic and cyclic ketones, e.g. acetone, ethyl methyl ketone, ethyl ketone, tert.-butyl methyl ketone, diisobutyl ketone, acetylacetone, acetonylacetone, cyclo-pentanone, acetophenone;
esters of aliphatic and cyclic carboxylic acids, e.g. triethoxymethane, methyl acetate, allyl acetate, methyl glycol acetate, ethylene glycol diacetate, glycerol-l-acetate, glycerol diacetate, methylcyclohexyl acetate, methyl salicylate, phenyl salicylate;
aliphatic and cyclic esters of phosphonic acid, e.g. methylphosphonic acid dimethyl ester, allylphosphonic acid diethyl ester;
aliphatic and cyclic oxyalcohols, e.g. 4-hydroxy-4-methyl-2-pentanone, salicyl-aldehyde;
aliphatic and cyclic aldehydes, e.g. acetaldehyde, propanal, trimethylacetaldehyde, crotonaldehyde, glutaraldehyde, 1,2.5,6-tetrahydrobenzaldehyde, benzaldehyde, benzene-propane, terephthalaldehyde;
aliphatic and cyclic oximes, e.g. butanone oxime, cyclohexanone oxime;
aliphatic and cyclic amines (primary, secondary or tertiary), e.g. ethylamine, diethyl-amine, triethylamine, dipropylamine, pyrrolidine, morpholine, 2-amino-pyrimidine;
aliphatic and cyclic polyamines (primary, secondary or tertiary), e.g. ethylene-diamine, 1-amino-2-diethylaminoethane, methyl-bis-(2-methylamino-ethyl)amine, permethyldiethylenetriamine, 1,4-cyclohexanediamine, 1,4-benzene-diamine;
aliphatic and cyclic hydroxyamines, e.g. ethanolamine, 2-methylethylamine, 2-methylaminoethanol, 2-(dimethylamino)ethanol, 2-(2-dim ethylamino-ethoxy)-ethanol, diethanolamine, N-methyldiethanolamine, triethanolamine, 2-(2-aminoethyl- amino)-ethanol, triisopropanolamine, 2-amino-2-hydroxymethyl-1,3-propanediol, 1-piperidine-ethanol, 2-aminophenol, barbituric acid, 2-(4-aminophenoxy)-ethanol, 5-amino-l-naphthol.
Suitable antioxidants are compounds of formulae (I), (II) and (III). 
wherein
R1 denotes an alkyl which is optionally substituted,
R2 denotes an alkyl which is optionally substituted or an aryl which is optionally substituted, and
n denotes 0 or 1,
preferably those in which at least one of the R1 and R2 radicals contains at least one xe2x80x94OH, xe2x80x94COOH or xe2x80x94SO3H group; 
wherein
R3 denotes an alkyl or acyl group; 
wherein
R4 denotes an alkylene group which is optionally interrupted by 0 atoms, and
m denotes a number of at least 2.
In addition to the aforementioned types of substitution, the alkyl groups R1, R2, R3, the alkylene group R4 and the aryl group R2 can also contain other substituents.
Examples of suitable antioxidants include 
The preferred solvents are alcohols, glycols, polyglycols and caprolactam, and optionally mixtures thereof also. Preferred p-phenylenediamine derivatives are listed in EP 0 980 024, paragraph 28.